Electron source for corpuscular radiation apparatus operating on a pump including an indirectly heated cathode



3,263,109 ATUS July 26, 1966 KARL-HEINZ HERRMANN E ELECTRON SOURCE FOR CORPUSGULAR RADIATION APPAR OPERATING ON A PUMP INCLUDING AN INDIRECTLY HEATED CATHODE 2 Sheets-Sheet 1 Filed July 19. 1962 July 26, 1966 KARL-HEINZ HERRMANN ETAL 3,263,109

ELECTRON SOURCE FOR CORPUSCULAR RADIATION APPARATUS OPERATING ON A PUMP INCLUDING AN INDIRECTLY HEATED CATHODE Filed July 19, 1962 2 Sheets-Sheet 2 United States Patent M 5 1 Claim. (Cl. 313-83) The invention disclosed herein is concerned with an electron source with indirectly heated cathode, for corpuscular radiation apparatus operating on a pump.

Corpuscular radiation apparatus operating on a pump often require electron sources which permit operation with high beam current strength. This is for example the case when using an electron source which is to serve for the heating of specimen by electron bombardment in an electron diffraction device.

However, further and in part contradictory requirements are posed for such electron sources. Thus, the electron sources shall on the one hand produce a high radiation current strength, while on the other hand a cathode temperature as low as possible shall sufiice for the production of the electron beam, so that the environment is only slightly heated by heat conduction and heat radiation. Moreover, a stable mechanical construction is required so as to avoid, upon thermal loading, changes in the position of the emitting cathode surface and therewith alteration of the direction of the emitted electron beam. In addition, electron sources for corpuscular ray apparatus operating on a pump, for example, electron microscopes, must be constructed of materials which can withstand aeration of the apparatus and which react as little as possible with operationally unavoidable hydrocarbon vapors, which are for example produced by vacuum gaskets and by fatty matter employed.

The present invention which is concerned with a corpuscular radiation device operating on a pump, proceeds from an indirectly heated cathode which can be in known manner constructed very stable. The above enumerated requirements are fulfilled by an electron source which comprises, according to the invention, a planar cathode made of a noble metal, for example, platinum, forming the emission surface, and a coating provided upon the emission surface, for example, an alkaline earth oxide coating, which reduces the electron emission work.

Planar cathodes which have become known for use as electron sources in electron-optical arrangements, are made of molybdenum or tungsten, the emission surface being coated with an alkaline earth metal which is by diffusion delivered from a supply chamber. Aside from the fact that arrangements of this kind do not permit obtaining in given cases sufficiently high beam current strength, there appear difficulties after an aeration of the apparatus. There is after each aeration a prolonged preheating time required before the electron emission assumes again a sufiicient value.

The use of platinum for cathodes or corpuscular radiation apparatus operating on the pump has become known until now only for point cathodes. Such cathodes do not permit achieving high radiation current strength such as, for example, required for the heating of massive specimen in vacuum. The beam current strength produced with the electron source according to the invention, lie higher by some orders of magnitude. The known platinum point cathodes are moreover directly heated.

The described difficulties experienced with the known arrangements are avoided by the provision of an electron source having a planar cathode adapted to produce high 3,253,109 Patented July 26, 1966 radiation current strength, the cathode being made of noble metal which has great reaction inertia. A high radiation current strength is moreover achieved, with avoidance of an impermissibly high heating, by the use of an oxide cathode.

The electron source made according to the invention is advantageously used in a Pierce system so as to obtain focusing of the emitted electron beam. The emission surface of the cathode is in such a case arranged in the central bore of an electrode which is at cathode potential. The emission surface is advantageously curved so that the emission plane merges with the surface of the curved electrode of the Pierce system.

The emission plane of the electron source is, in a particularly advantageous construction thereof, formed by the end surface of a pot-like tubular member made of noble metal surrounding the heater helix which may be molded in place in known manner. The tubular member of noble metal is fixedly disposed with-in a holder sleeve made of poorly heat conducting but electrically good conducting material, for example, tantalum, such sleeve also forming cathode terminal means.

It may be frequently necessary to adjust the emission plane at least in directions extending perpendicularly to the system axis. The sleeve, which holds the tubular member of noble metal, is for this purpose arranged upon auxiliary elements which are disposed on a ceramic base, such elements permitting, for example, with the aid of screws, centering of the sleeve and therewith centering of the emission plane.

The ceramic base is preferably made in the form of a plug with terminals for the supply of heater voltage, thus permitting in simple manner, upon opening of the apparatus, removal of the electron source for the renewal of the coating which reduces the electron emission work. This substance is, for example, an alkaline earth coating which is provided in the form of an alkaline earth carbonate and which is in operation converted into the corresponding oxide.

Further details of the invention will now be described with reference to the accompanying drawings, showing an embodiment thereof.

FIG. 1 shows an embodiment of the new electron source in relation to apparatus parts which are indicated in cross section;

FIG. 2 shows part of the arrangement in elevation; and

FIG. 3 represents in cross-sectional view, on an enlarged scale, the cathode and related parts.

Referring now to the drawings, numeral 1 indicates the tubular member made of noble metal, forming the end plane 2 which represents the emission plane or surface provided with a coating 3, for example, an alkaline earth oxide coating, which reduces the electron emission work, such member 1 being held in place by a sleeve 4. The connection between the noble metal member 1 and the sleeve 4 which may be made, for example, of tantalum, is effected by spot welding. The use of the sleeve 4 offers the advantage of providing a reliable mounting for the member 1, with good heat insulation, and of facilitating the fabrication. The sleeve 4 also serves as a cathode terminal.

The sleeve 4 is, with the aid of auxiliary elements yet to be described, arranged upon the ceramic base 5 which is provided with tubular plug pins 6 and 7 for supplying the heater voltage. Between the sleeve 4 and the ceramic base 5 are disposed a ifianged member 8 which is made for example, of nickel, and a metal socket 9. Laterally extending screws 10, 11, 12 projecting through the tubular metal bushing 13, are provided for centering, with respect to the system axis, the socket 9 and therewith the cathode with the emission surface 2.

emitted from the emission surface 2 of the cathode, the

surface 17 of the anode 1 which faces the electrode 14 being complementarily curved as indicated in FIG. 1. The emission plane 2 of the cathode is arcuately shaped (see FIG. 3) so that it merges smoothly with the concave surface 16 of the electrode 14.

The electrode 14 has been omitted in FIG. 2 to simplify representation.

As is apparent from FIG. 3, the tubular member 1, made of noble metal, surrounds the heater helix 18, such helix being in suitable known manner embedded in a known molding mass, for example, in a sinter cement 19. The terminal wires 20, 21 of the heater helix are extended through the plug pins 6, 7, as shown in FIG. 1.

The tubular noble metal member 1 is held by the surrounding sleeve 4 which is made of poorly heat conducting by electrically good conducting material, for example, tantalum. This sleeve, which is held in the flanged tubular member 8, also reduces heat radiation.

It will be appreciated that the invention is not inherently limited to the illustrated embodiment. Important for satisfying the requirement initially posed for an electron source which is to be used for a corpuscular apparatus operating on a pump, is the embodiment of the cathode, made in accordance with the invention, constituting an indirectly heated planar cathode made of noble metal and provided with a coating which reduces the electron emitting work.

Changes may be made within the scope and spirit of the appended claim which defines what is believed to be new and desired to have protected by Letters Patent.

We claim:

In an electron source for use in corpuscular radiation apparatus operating on a pump, the combination of a bushing, a ceramic base disposed in said bushing, a socket within said bushing and supported adjacent said base, a plurality of centering screws extending through walls of said bushing and engaging said socket, a flanged member mounted on said socket, a sleeve supported by said flanged member, a generally pot-like tubular member of noble metal supported within said sleeve, the closed end of said member extending from one end of said sleeve and defining a cathode and forming an emission plane, a heater helix disposed in said tubular member and having terminals extending through said socket and said ceramic base, a coating provided on said emission plane which reduces the electron emission work, an electrode secured to said bushing disposed at the same potential as said cathode and having a bore therein receiving the closed end of said cathode, a surface of said electrode and the emission plane each being arcuate and positioned to be complementary with one another, said centering screws disposed for centering said cathode within said bore.

References Cited by the Examiner UNITED STATES PATENTS Bondley 313337 DAVID J. GALVIN, Primary Examiner. 

